This invention relates to chain saws. More particularly, this invention relates to an electronically actuated chain saw safety brake mechanism for reducing the likelihood of injury to the user of the chain saw, which might result from the uncontrollable phenomenon known as "kick-back."
A well known potential hazard that is always present when using a chain saw is "kick-back." A "kick-back" can be produced by a chain saw when the cutting elements of the chain momentarily stop cutting and are seized by the work material. The kinetic energy of the chain normally used to remove the material is suddenly transfered to the chain part, imparting a force to the chain bar that causes it to kick up toward the operator. The magnitude of the kick-back force is related to the speed of the chain, and the nature of the engagement of the chain with the work material.
One known way for potentially reducing the likelihood of injury as a result of kick-back, involves placing a manually operated hand guard actuator in a position to be contacted by the back of the hand or wrist of the operator in the event of a kick-back and thereupon actuate a chain brake device manually to apply the brake and stop the saw chain. The clearance between the front handle and the brake actuator necessary to allow convenient operation of the chain saw requires a large angle of rotation of the saw above the wrist before actuation occurs. This large angle of rotation allows the driven chain to approach the user a considerable distance before actuation of the brake occurs. Additionally, the operator in attempting to protect himself may remove his hand from the hand grip without actuating the brake. Another possibility is for the user to firmly grip the chain saw during the kick-back so that the user's arms and chain saw act as a rigid body rotating about an external point. In this case the relative motion between the hand guard and the user's wrist necessary to actuate the brake may never occur. An example of this type of mechanism may be found in U.S. Pat. No. 3,839,795. Examples of other mechanisms proposed or which are actually put into use for reducing the likelihood of injury as a result of kick-back may be found in U.S. Pat. Nos. 3,923,126 and 3,958,680. Examples of other patents considered pertinent in one way or another to this invention are as follows:
3,485,326--Wilkin PA1 3,485,327--Gudmundsen PA1 3,739,475--Moore PA1 3,765,516--Thellufsen PA1 3,785,465--Johansson PA1 3,793,727--Moore PA1 3,810,309--Wiklund et al. PA1 3,857,180--Dooley
Notwithstanding the advances in the chain saw art, there is always room for improvement. In this connection it seemed to applicant beneficial to provide a method and apparatus which is responsive to a kickback regardless of whether or not the saw operator's hand slips from the saw handle in a special way because his or her hand has to be placed at a strategic location on the handle means employed. A kickback is seen to relate to the vertical movement of the cutting chain over the guide bar nose upon engaging a soft spot or knot, inducing the bar to kick upwards abruptly.
While others have provided an inertial responsive system for operably disengaging a chain saw drive shaft from the chain saw chain in response to an abrupt upward swing of the guide bar generally around the centre of gravity of the chain saw, applicant is believed to be the first to successfully brake the unit to a stop, in addition to the normal disengaging process, stops the chain.